January in Aserrí, Costa Rica, brings average daily highs of 24°C and overnight lows of 16°C in the 2016–2025 historical record. Pack clothes you can layer as conditions change. The average leaves room for both warmer and cooler days, so a flexible walking plan is more useful than deciding every outdoor stop before you arrive.
In the record, at least 1 mm of rain or snow falls on 6 of 31 days on average, a daily chance of 21%. Leave some freedom to swap an outdoor day with a sheltered one. There is no need to read the average as a fixed number of lost days; it says nothing about how long rain lasts on each occasion.
The calculated day length, sunrise to sunset, averages about 12 hours. Sunshine averages 11 hours a day. Use the daylight window to plan the length of excursions, and check local sunrise and sunset times for the dates of your stay. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
Start with the places you most want to visit, then keep the order flexible enough to suit the weather. Check current opening hours and local transport before fixing long day trips. The climate record can help with pacing and packing, but it does not describe what is open during your stay.
The following month, February, has a similar average daily high of 24°C. Its historical rain-day chance is 17%, compared with 21% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.